The present invention relates to a surface acoustic wave filter device which effectively serves as a very small filter for use in electronic circuits.
A surface acoustic wave filter device comprises a filter chip providing an input transducer and an output transducer having forms which each comprise a pair of comb-shaped electrodes on a piezo-electric substrate, and a case which accommodates a filter chip. A radio-frequency voltage is applied to the input transducer which generates acoustic waves, and the acoustic waves thus generated are received by the output transducer and are converted into electric signals, so that a filtering function such as band-pass is embodied by means of the input and output transducers. The application of the input voltage to the input transducer is carried out by connecting a wire to an externally leading electrode of the input transducer via an external input signal leader terminal attached to the case, and the transmitting out of the output voltage from the output transducer is carried out by connecting a wire to an external output signal leader terminal attached to the case via an externally leading electrode of the output transducer. When such a surface acoustic wave filter is used for a television receiver, especially for an intermediate-frequency circuit of a television receiver, only signals within a particular frequency band, determined by the shapes of the electrodes of the input and output transducers, are allowed to propagate from the input transducer to the output transducer, i.e., only signals of an intermediate frequency are allowed to propagate in the form of surface acoustic waves from the input transducer to the output transducer. In practice, however, it was revealed that signals produced at the output terminals (which include the external leader terminal attached to the case for the input signal, the externally leading electrode of the input transducer on the filter chip, and the wire connected between said externally leading electrode of the input transducer and said external leader terminal) of the surface acoustic wave filter device, within a television receiver, contained signal components which had not assumed the form of acoustic surface waves, but which had been directly transmitted by inductive coupling. The induced field directly propagates from the circuitry external to the surface acoustic wave filter device to the output terminals (which include the external leader terminal attached to the case for the input signal, the externally leading electrode of the input transducer on the filter chip, and the wire connected between said externally leading electrode of the input transducer and said external leader terminal) of the surface acoustic wave filter device due to the principle of electromagnetic induction. When the output terminals of the surface acoustic wave filter are connected to the input terminals of a differential amplifier, the difference in the level of signal components induced on the two input terminals becomes a problem. Namely, when the levels of inductive coupling in the two input terminals of the differential amplifier are the same, the signal levels cancel each other through the differential amplifier and no difference appears in the output. However, when there is a difference in the levels which are induced, the difference is not cancelled but appears in the output. That is to say, an increase in the induced components results in increased ripple in the pass-band characteristics of the surface acoustic wave filter device, so that the attenuation of the rejection band decreases and the characteristics of the filter deteriorate.
To eliminate the defects caused by induction, as described above, the whole surface acoustic wave filter device, including the filter chip and the input and output terminals of the chip, should be shielded. The filter chip can be shielded if the filter chip is housed in a metal case, but the external leader terminals which are portions of the input and the output terminals cannot be completely shielded, because the external leader terminals must protrude from the metal case for the purpose of connecting the filter device to the external circuit. Further, if the external leader terminals are shielded by using a shield plate, etc., the number of the parts used in the device increases and the assembly of the parts is troublesome, so that the cost of the device increases. Further, since the induction is developed not only by the external terminal leader but also by various other members, the above attempt at shielding encounters a variety of difficulties.